Control of two-way signal transmission systems



K. H. DAVIS Aug. 15, 1944.

CONTROL OF TWO-WAY SIGNAL' TRANSMISSION SYSTEMS Filed Sept. 17, 1942 2Sheets-Sheet 2 VINVENTOR' K ht DAV/S ATTORNFV Patented Aug. 15, 1944CONTROL OF TWO-WAY SIGNAL TRANSMISSION SYSTEMS Kingsbury H.BernardsvillflN. J., assignor to Bell Telephone Laboratories,Incorporated, New York, N. Y; ia'c'orporation of New York r ApplicationSeptember 17, 1942; Serial No. 458,658 g I 4 Claims. 1(c1,17s; -44)i Theinvention relates to two-way signal transmission systems andparticularly to the signal-' controlled switching circuits used withsuch systems to directionally control signal transmission therein whilesuppressing echoes and preventing singing.

Such signal-controlled switching circuits, commonly called echosuppressors, usually employ two switching branches respectivelyconnectedto each of two oppositely directed one-way signal transmissionpaths at a terminal'or intermediate point in the two-way system, eachincluding a rectifie'r for rectifying a portion of the signals divertedinto the switching branch and electromagnetic relays or other switchingdevices op erated by therectifled signals to suitablycontrol, thetransmission efficiency of the signal transmission paths and todisablethe other switching' branch so as to provide the desired direc tionalcontrol of signal transmission while suppressing echoes and preventingsinging.

The copending patent application of K. H.

Davis and A. C. Norwine, Serial No. 421,704, filed December'3,'1941,which issued as Patent No.

2,306,689 on December 29, 1942, discloses such an' echo suppressorapplied to a four-wire telephone circuit, modified so as to enable onesubscriber by talking loudly to break in on another who is talking andhas obtained directional control of'the system, while the lattercontinues totalk and'regardless of how loudly the latter party is"talking. This is accomplished by the use or relaftively insensitiveauxiliary switching" controls, properly located with=-respect to theecho suppressor controls, operating on an amplitude basis to disable theecho suppressor controls of the talking party when the breaking partytalkswith the required loudness.

An object of the invention is to improve break in echo suppressors oftheabove-described type.

A more specificobject is to provide echo, sup-(.1.

pression and to enable break-in in systemsof the above-described typewith economyof apparatus.

These objects'are attained in accordance with the invention'byarrangements for automatically transforming the echo suppressinequipment as! sociated with each side of a four-wire circuitintobreak-in equipment during periods in which it is not needed forerforming itsecho suppressing f functionl Fig.1 showsschematically afour-wire toll te1e-' N2, respectively.

phone circuit equipped with an echo suppressor of the amplitude breakintype embodying one modification of the invention; and

Fig. 2 shows schematically in more detail an :alternative echosuppressor of the amplitude break-in type in accordance with theinvention which may be used in the system of Fig. 1.

InlFign'l, the four-wiretele'phone circuit in-.

cludes a'one-way,- two-wire transmission path .EA' including theamplifier As, normally conditioned totransmit in the direction from westto eastbetween the two-way line WL and the twowayline EL; and a one-way,two-wire transmission path .WA including theone-way amplifier Aw,normally. conditioned totransmit. in the dienergy transmitting relationwith the two-way lineJWL or EL by the hybrid coil Hw and associated linebalancing network. Nw, or hybrid COil-HE and associated line .balancingnetwork Connected across the path EA at an intermediate point I is theinputof a switching circuit 2, operating as a combined echo suppressorand break-in, control circuit, including the one-way amplifier 3=followedby the'wave rectifier (detector) 4 and the electormagneticswitching relays, 5 to 8 adaptedto be operatively-energized in parallelby the rectifiedsignal output of the rectifier 4., Similarly, connectedacross the path WAtatan intermediate point 9 is the inputof aswitchingcircuit 10, operating as a combined echo suppressorand break-incontrol circuit, including the one-wayamplifier ll followed by the waverectifier (detector) "l2 and the electromagnetic switching relaysu|3 toIE adapted to be operatively energized in parallel by the rectifiedsignal output of the rectifier l2.

.In theswitchingscircuit 2, relay 8 to be referred to hereinafter as thewest break-in relay, is relatively insensitive compared to the otherswitching relays 5 to l in that circuit, as indi catedzby the seriesresistances IT in the energizing circuit forthe former relayi only.Also, theenergizingcircuit for the relay 8i is normally disabled by anopen circuit at thepoint l8 so that the relay is normally not'incondition-to be operated bythe rectified signal output ofrectifi81::4,,Wh81eaS the energizing circuits for the relays '5.tolarenormally operative so that the latter relays maybe immediatelyoperated bya be'transmitted back over that path. I

rectified signal output from the rectifier 4. Similarly, in theswitching circuit In the relay Hi, to be referred to hereinafter as theeast breakin relay, is relatively insensitive compared to the otherrelays l3 to l5 in that switching circuit, as indicated by theresistances IS in the energizing circuit of the jormer relay only, Also,the I energizingtcircuitgfor the break in relay I6 is normally disabledby a normal open circuit at the oint so that normally the relay is notin condition to be operated by signal output from" the rectifier l2,whereas the energlzingrcircuits for the relays l3 to IE are normallyoperative so that these relays are in condition to be immediatelyoperated by the signal output the rectifier I2.

Relay 5 in the switching circuit to' b e referred to hereinafter as thewest echo suppressor relay i when operated, connects a short-circuitingconnection across the path WA at a point 2| beyond the point Soto whichthe input of the switching circuitiifi is connected, to disablethatpath. Similarly, the relay l3 in switchingcircuit llLtobereferredlto hereinafter as the east echo suppressor .relay whenoperated, closes a short-cir the;energizing circuits for the relays l3to l5 ,aofthe eastswitching circuit It), so as to prevent cuitingconnection' acrosszthe path EA at the. point 22beyond the point]to-whichthe input a of; theiswitching-circuit 2 is connected, todisable-the latterpath. .The one-.way-amplifier As is connected in thepath EA between the pointsl and: 22 to prevent the disabling of thatpath from affecting the input of the switchingcircuit 2.-Simi1arly,"ithe one-wayamplifier Aw is con nected inthe path'WAbetween'the/points 9 and" 2| to preventfthetdisabling of-the path WA fromaffecting the-input to the switching circuit-I0.

In the following description of operation of the system of; Fig. 1, itwill be convenient to speak of a loud talkerfi meaning one who. can

can' operate. a 'breakin control and a 'weak' talker, :meaning' one whocannot. Since we are to deal with circuits at the terminals 'ofwhichportions of signals are returned asechoes, it is as'sume'dthat aloudtalker in order to break-in must operate a control at least insensitiveenoughsothatit will not operate on the echo of the other talkers speechenergy Let it be assumed that a West telephonesubscriber :associatedwith the line:WL starts to talk, and at that'timeithe ea'stsubscriberassociated" with theilinexELdsi silent; Thespeech wavesjof the;westcsubscriber received 4 over the line WL are impressed by the hybridcoil Hw=on the path. 'EA :of the four-wire circuit and, that Ipath-.lbeinglin its normally operative condition with-theshort-circuiting connection at the point 22 -open,= will betransmittedthereOVer amplified byv-the-amplifier AE to' 'the'ea's't'terminal of'the' four-wire circuit, where the'rnain'portion willbe transmitted through the'hybrid coil HE to the" line-EL leading to theeast subscriber, and'an other portion will be reflected 'as echoes intothe oppositely'directed" one-way'path WA and will Alportionof Westsspeech signals willbe' diverted at thepoint I in path EA- into theswitch- 4 ing circuit Land will be amplified by amplifier 3 andrectifiedtby the'wave rectifier 4. The rectified signals inthe output ofrectifier 4wil1 cause the immediate; operation of: the. switching relays5 32. lnbll i. regardless of their amplitude, will not-be ableto;:operateithebreak-iri:relay 8 be- 319.5? of t ;nQ fm l iin perativeenergizing cir.-

u c lli connec on across. the path WA: at the ;R- Will Qperateto closethe short-cirsubsequentoperation of those relays by operati'on'ofthewave rectifier I2 in response to later jsignals' offthe eastsubscriber in the path WA, while'thewest subscriber continues to talk.Relayl will-simultaneously operate to close the open circuit at thepoint 20 inthe energizing circuit for'the east break-in relay I6 in theeast switching; ,c ircuit I0, soas to enable the latter relaytobethereafter operated by any rectified signals in the outputof thewave rectifier l2 ofsufficient amplitude .to overcome ;thedesensitization provided by-the resistances IS in itsenergizing circuit.The west subscriber now has obtainedexelusive directional control of theecho suppressor circuitsand has conditioned those circuits so as toenable break inbythe. eastsubscriber.

Now, it While Westis still talking, East desires,

todnterrirpt him, the latter will start to talk and-,ra ise his voicesothat at least a portion of his speech signals will have an amplitudesufficientto overcome the desensitization of the east break-in relay l5provided 'by theresistances H! in its energizing circuit. 'lhen theportion of Easts higheramplitude, speech Waves diverted into theswitching ,circu itglfl from the path WA willl be amplified by amplifierH and rectified by rectifierv -lzand will cause the operation of theeast break-in-relay 6,4120 open the energizing circuit for ,the westechosuppressorrelay 5 only.

The consequent release of the west echo suppressoifvrelay 5 to breakthefshort-circuiting connection across the path WA at the point 2|willenable that path so as to allow E'asts speech waves 'to.pass over,the output of that path through hybrid coil Hw to line WL and over thatlineto thewe st subscriber.

If the westsubscriber continues to talk, the rectifier}! iniswitcliing,circuit 2 will be maintaine" operatedby; his speech currents to holdrelays Band I operated tomaintain'the energizingcircuit fortheeastbreak-in relay l6 operativej and theenergizing circuits for the otherrelays ;I3,-,IQ- and l5 in theeastswitching circuit l0 disabled, Thepath EA; therefore, will be also maintained unblocked so that bothsubscribers will'hear eachrother, but there will be no echo vsuppression and both subscribers will hear echoes.

This condition 'will continue until the loud talker (East) lowers "hisvoice sufiicie'ntly to cause the east break-in relay 1 6 to releaseandthus allow the westecho suppressor relay 5 tob'again operated by West'tblock the path WA attne'poi t 2 l giving "directional controlof thecircuit: again tq' 'th'efjlatter, or until the weak talki-(Wstif ops" tl ng. in; the latter case. the wa s 5 and'l'ofthe switching circuit 2will relaseand ewire e s l3 l4 a ite b pe e b East to iY reQ Qnalo contl Qtt e circui tQ-him, dtocondi n th west, breakr n e ay. 8 .for 1subse'quentjoperation by;West-,if the latter again I starts talking withsufiicie'nt: loudness to 'over-.

come the desensitization of that relay provided bythe resistances l1. i

Now, ifwhile East is-talking loudly, West also raises his voice .to aloudness suflicient to over-.

come the desensitization of the :west break-in relay 8, there willbe nochange in theoperation since, because the energizing circuit for therelay EA and WA will remain. enabled and both: talkers willhear eachother and echoes during the period of double talking. v n

Similarly, if the east subscriberstarts talking first, he willobtaindirectional control of the circuit by operating switching circuit[0:130 disable the path EA at the point .22; to disable the energizingcircuits for theJrelays-S, .6 and I of the west switching circuit 2, andto render the ener- .To make the above-describedtype of break-in Yeffective, it isnecessarysthat the various echo suppressor and break in.controls have their rela- The tive sensitivities properly proportioned.values of the resistances l1 and I9 in the energizing circuits for thewest break-in relay 8 and the east break-inrelay l6,respectively,:should beselected so that when the energizing circuitsiorthese relays are operative, the break-in control wi11 be madesufficiently insensitive so that each break-in relay will not beoperated in response to the maximum amount of signal echo which mayoccur in the associated signal trans mission path. -Also, the severalswitching relays should be provided with sufficient 'hang-oversinoperation to prevent [their premature release to cause clipping or allowechoes. i Fig. 2 shows aportioniof. a four-wi native type of amplitudebreak-in echo suppressor in accordance with the invention. Asshown, thehybrid transformer H1: .is inserted inthe 'west to-east path EA of'thefour-wire'circuitat an intermediate point corresponding to the pointl ofFig. l, anda hybrid transformer H2 isconnected in the east-to-west pathwA at an inter normally closed back contacts of the unoperated east echosuppressor relay.'24,;and the hybrid transformer H2 comprises two linewindings con nected in the path. .WA anda third winding 25} 212); gizingcircuit for the westbreak-in relay 8 operative to enable West tobreak-in subsequently bytalking loudly enough to overcome thedesensitization of that relay provided by the resistances IQEQCathOdGOiI'CUlt of the direct current amplifier 32;

break-in control circuit 27, to be referred to hereinafteras the westswitching circuit, including in 7 order: the vacuum tube amplifier 28;the two-v elem'entvacuum tube detector 29; the west breakin .masterrelay 30 controlling the operation of th .1west-;break-in' relay 3| andits associatedhang-over circuit; the direct current vacuumtubefamplifier 32; the west master switching re-i layj 33;. and, thewest echo suppressor relay 26.

with its associated hang-over circuit. .Similarly,

connected across the path WA at a point be tween the. line windings ofthehybrid. trans former. HaiS the input of the combined echo sup-Hpressor and break-in control circuit 34, to be rergferredto hereinafteras the east switching .cir-

cuit, including in order: the vacuumtube arn'pliav fier.35'; thetwo-element vacuum tube detector- 36;.the east break-in master relay 31controlling the operation of the east break-in relay 38 and rent vacuumtube amplifier 32, the east master ;its associated hang-over circuit;the direct-curswitching relay 39 having itsoperating winding connectedin series with the operating winding of the west master switching relay33 in the plateand the eastzecho suppressor relay 24 :The westtwo-element vacuum tube detector Y 29 and the east two-element vacuumtube demgtectorBG, which may as shown, be containedin e telephone'circuit like that of "Fig-l equipped withanalterinductively coupled'theretox'which *is' normally" short-circuited through the normallyclosed back contacts of thelunoperatedwest echo suppressor relay 26. Inthe normalconditiori'ofthe'hybrid transformer H1 With-its thirdlwinding23 short missionover the pathWA.

. condition of theh brid transformer Hz with its third winding 25short-circuitedthe latterv trans y 70' former is unbalanced so as topermitsignal trans:

Connected; across the path EA. at a point be- 15 tween. the. linewindings of hybrid transformer 5 it P 1 1491 293?! Qmbinedechosuppressor a w a single tube, are oppositely poled with respect to;the control grid-cathode circuit of the direct current amplifier 32.Eachis also biased in the non-conducting direction by batteries 56 and51;

r srespectively; The .west master switching relay" 33 is, more sensitivethan the east master switch ing relay 39 so" that the former is heldoperated by the normal platecurrent (current with no voltage-,a'ppliedto its control grid from either 4oi-;detector) of the direct currentamplifier 32 flow:

ing through its operating winding and the oper ating winding of the eastmaster switching relay 39, this normal plate current not beingsufficient to operate the latter relay the armature-of which is.maintained on the back switching contact. ""This normal plate current isdetermined by the difference. in the selected voltages of batteries and58. w

The other elements of theecho suppressor cirdescription. of operation.

cuitsof. Figo2 will be referred to in the following is, speaking, and atthat time the subscriber at thepeastterminal of the four-wire telephonecircuit.,is silent. Because of the normal unbalanced condition 'ofzthehybrid transformer Hi, the west subscribersspeech waves will pass freelyover the path. EA .with the amplification provided by amplifier Asandxother amplifiers (not shown) togthe line leading to the eastsubscriber; and at the {east terminal of the four-wire circuit aertiomofj, these waves will be reflected as echo-cur rentsinto theoppositely directed transmission" at W ;A, portion of verted from thepath EA' at a point between the line windings of the hybrid transformerHr into Wests speech waves will be dithe West switching circuit 21 andwill be amplified insensitive'so that it will not be operated by'the Ipyithe amplifier 2a andrectifiedfby the detector;

rectified signals of normal amplitude passing through its operatingwinding from the output of detector-29. The rectified signal. currentpasses through the winding of the latter relay, the.

2-megohm resistance 40 and the input. potentiometer 4| for the directcurrent amplifier 32 in such direction as to apply a negative bias tothe control grid of that amplifier thus reducing' the plate current ofthe latter. The reduction; in

the plate current of the directcur'rent' amplifier 32 :flowing throughthe operating winding of the west master relay 33' and the operatingwinding of the east master relay 39 miseries-will cause the-former moresensitive relay 33-:to release to 42.- The-make of the back switchingcontact ofrelay 33 will cause the condenser of the resistance-condenserhang-over arrangement 43; normally charged by current; from thel batteryM through the hang-over winding of Vthelrelay 26,

to be discharged to ground, so that when West ceases talking the releaseof the west echo"suppressor relay 26 will be delayed for a desired.

hang-over interval determined by the'tim'e' rer quired for thatcondenser to be charged iup'to'v the potential of battery 44throug'h'thehangoverwind-ing of relay zfi when relay 33 returns: 1

t -its normal operated condition, the current through the hang-overwindin energizing'thati winding to hold the relay operated.

-When the west echo suppressor relay. 26 oper-' ates it will break itsback switching contact: and

make its front switching contact. The break of the back switchingcontact will open the normal short-circuit across the winding 25 of thehybrid transformer H2 in the path WA causing. that hybrid transformer tobe balanced and thus efiectively insert a disabling loss into the pathWA which will suppress the echoes of .Wests speech waves when theyarrive at that point in the path.

The make of the front switching contact of relay 26 will cause thenegative battery 35 to be connected to the upper terminal of the'potentiom-r eter through the 2-megohm resistance 46. As

an equivalent negative battery 41 is' connected to the lower terminal ofthat potentiometer, this will effectively isolate. the output of theeast de-: l

tector 35 from the input of the direct current amplifier 32 whileretaining an operative connection of the east break-in master relay 3'!in series with battery 45 across the output of the east detectorsfithereby conditioning the circuit NQW, if, while West is still.talking, theeast forbreak-in by the east subscriber.

subscriber desires to interrupt him,rthe latter start talkin and willraise his voice sufficiently so that the portion of "his speech wavesdiverted from the east to west path WA at thehybrid amplified by: theamplifier 35.- and rectified by the, detector 36 therein, will be ofsufficientamplitude to cause the operation of the east break-in masterrelay 3] in the output of that-detector. The operation of the latterrelay to H close its switching contactswill cause the operatingwindingto'i the east break-in relay 38 to be supplied with energizingcurrent from the battery 49,;and

will cause the condenser oi the associated hangcoil H2 into the eastswitchingjcircuit 34 and overcircui't 501 normally charged from battery"49, to be discharged to ground. The consequent.

operation of the east break-in relay 38 will conmet the negative battery5| to one side of the winding 25 of the hybrid' c'oil H2. Asanequivalent negative battery 45 is. connected to the other side of thewinding 25, that winding isthus effectively short'ecircuited tounbalance the a hybrid coil H2 in the path WA allowing Easts speechwaves to be transmitted out over that path to the west subscriber.

The isolation the utput of the east detector- 36 .fromthe input of thedirect current ampl g t fier 32 maintained by operation of the westecho'suppressor relay 26 in response to'Wests' speech waves in the mannerwhich has been described; I will preventEast'from causing the operationof the east master switchin relay 39 in the output of that amplifier,while West continues to talk; 'I'husthe 'east echo suppressor relay 24for that condition will-be maintained in its normally unoperatedcondition to hold former Hlin the path EA unbalanced to allow the hybridtrans Wests continuing speech waves to be transmitted over that path tothe east subscriber. Thus, both subscribers. will: hear each otherduring the pe riod of-double talking, but there will be no echosuppression and both parties will hear echoes; This condition willcontinue. until the loud talker (East) lowers his voice sufiiciently tocause the" release of the east break-in master relay 3'! and thusfinally the release of the east break-in re' lay 38'to remove thenegative battery 5| from i one side of the winding '25'0f hybrid coil Hzto 1 remove the e'fiectiveshort circuit from that wind-- ing. This willallow West to regain directional control of the, circuit by continuedoperation of the west,echo suppressor relay 26 to hold its back contactopen. @Therelease of the east breakin relay38, of course, will bedelayed for a given hang-overt interval required for battery 49 tocharge up the condenser of cult .59 through the winding of relay 38after the east master relay 35*releases, thus maintaining thebreak-ineffective for normal pauses transmission of Easts speech waves.

in the Ifwhile East is talking loudly, the weak talker (West) stopstalking so that his relay 33 releases causing the release of the westecho suppressor relay 26 after theex-piration of its hang-over(completionwof, charge'of the condenser of hangover circuit 43, frombattery 44 through the hang over winding), the connection of thenegative battery 45 to the upper terminal of the potentiometer 4.! "fordirect current amplifier 32 will i be broken.

Thus the isolation of that amplifier" from the-output of the .eastdetector 36 will 5 cease.

Then the rectified'speech signals of East in, the output of the detector36 will apply through i the; windi of .relay3l,- resistance 46 and poten'tiometer 4|, a positive.

biasto the control gridof the direct current amplifierfltube 32 toincrease the plate current of that amplifier suincientlyso that thejeast-master relay 39 in its out- I putwill be operated and the west'masterrelay; in its normal operated 33 will be maintainedcondition. Theconsequent shiftingof'the armature of relay 3-9. from thebaek to thefront contact will causetheoperating-winding of the eastechosuppressor-relay I 2 1te -"Joe energized fro m' f battery 52andfIwill-causeth' condenser ofthecondenser-resistancehang-over'-arrangemerit 55' normallyv charged from battery '53 through'the hang-over winding of relay 24 to be'discharged i to, ground;through the made" front contact of tact will 'remove the normal'fsl'iortcircuit from the winding 23 of the hybridl coil'Hi inthe path L EA to,unbala ric'e .that hyb d t ransformer' soas to put" an echo suppressingoss into that? path.

ruse; the" make. of theiro'nt; fcontactfoffthe ref lay 124 will connectnegative battery 54 through the 2-megohm' resistance An s the ub l.termii Y nal ofv the input potentiometer 41" for'the direct currentamplifier 36". As "an equivalent negative battery 41 is alreadyconnected to the lower terminal of that potentiometer, this will causethe input of the direct current amplifier 32 to be effectively isolatedfrom the west detector 29,

while maintaining an operative connection of the west break-in masterrelay 30 in series with battery 54 across the output of that detector.Therefore, the relay 30 is in condition to be operated in response torectified signals of the west subscriber of sufiicient level (greaterthan normal level) to enable break-in by West in a manner similar tothat which has been described in the case of the east subscriber.

When East ceases talkin the east echo suppressor relay 24 will bemaintained operated to block the path EA for a desired hang-over timeinterval determined by the time required for the condenser of thehang-over network 55 to be charged up from battery 53 through thehangover winding of the relay.

Now, if while the west or east subscriber is talking loudly, the othersubscriber east or west also starts to talk loudly, there will be nochange in operation from the case where one subscriber is talking weaklyand the other loudly, because, in that case both master break-in relays30 and 31 will be operated to cause the operation of the west break-inrelay 3| and the east break-in relay 38, respectively, to unbalance therespective hybrid transformers H1 and Hz to render both paths EA and WAoperative.

Because of the conjugate relation of the third Winding 23 of the hybridtransformer H1 to the input of the west switching circuit H and theconjugate relation of the third winding 25 of hybrid transformer Hz tothe input of the east switching circuit 34, the disabling of therespective transmission paths EA and WA will not affect the inputs ofthe two switching circuits.

It will be apparent that because of the combination functions of certainof the controls of the system of Fig. 1, and of the system of Fig.2,that both echo suppressor and break-in may be obtained with the use of asmaller amount of appa ratus than would be required if these twofunctions were exercised by two entirely separate control circuits.

Various modifications in the circuits which have been illustrated anddescribed and within the scope of the invention will occur to personsskilled in the art.

What is claimed is:

1. In a two-way telephone sy WO PP sitely directed one-way transmissionpaths for respectively transmitting over the system the voice signals ofthe subscribers at opposite ends of the system, one switching deviceconnected to .each path having a plurality of switching branches withsome transmission apparatus in common, certain of said switchingbranches operating in response to the voice signals of one subscriber inthat path, in the absence of prior voice signals of the other subscriberin the other directed path, to disable said other ath 'andsaidcertainswitchingbranches of the otherswitchinggdeviceconnectedthereto, so as tog eff directionalfcqntrol of signal transmission over t e system it thesubscriber, wh first starts t talk, and. means; to facilitateinterruption of the" a i g-'- a i r-ywi n her. s bs r comprising 3'1normally disabled, relativelyinsensitive switching branch for eachswitchin device, which," when operated, disables ,the transmiss on' atdisabling function. n y oflthe other s'witchi'rig device and meansresponsive to operation of said one switching device connected to onepath by the voice signals of said subscriber who first starts to talk toimmediately render operative the relatively insensitive switching branchof the other switching device connected to the other path so as toenable operation of the latter branch in response to subsequent voicesignals of the other subscriber which are of sufficient loudness toovercome its desensitization.

2. In a two-way telephone system, two oppositely directed one-waytransmission paths for respectively transmitting the oppositely directedvoice signals, and a switching device connected to each path, consistingof a common signal detector operatively responsive to voice signaltransmission in the connected path, and a plurality of switchingbranches operatively controlled by the signal output of said detector,certain of said switching branches operating in re- 0 while the signaltransmission in the first path operation of the associated continues, inresponse to subsequently initiated voice signals in said other path,only of an amplitude level at least a given amount greater than normallevel, to cause said other path to be enabled, and to disable allfunctions of said other switching device except its transmission pathenabling function.

3. The system of claim 2, in which one of said plurality of switchingbranches of the switchin device connected to each pin, is relativelyinsensitive and is normally disabled, said one switching branch whenoperated in response to detector by applied voice signal transmission ofsuflicient amplitude level in the associated transmission path,disabling the tranmission path disabling function only of the otherswitching device, the transforming of the other switching device causedby operation of said one switching branch of the first switching deviceconsisting in removing the normal disability from the relativelyinsensitive switching branch of said other switching device.

4. The'system of claim 1, in which said plurality of switching branchesof the switching device connected to each path includes a common wavedetector operatively responsive to the direct voice signals in theassociated transmission path but unrepsonsive to the signal echoes ofmaximum amplitude produced therein, and individual relays in the severalbranches, the individual relays in said certain branches of theswitching device connected to each path being responsive to therectified voice signal output of the detector in that switching device,in the absence of a prior rectified signal output for 6 was the detectorin the other switching device connected to theother path, to insertanecho suppression loss in said other path, and to effectively disablethe individual relays .in said certain branches of 'saidjother switchingdevice, the v relay in the other switching branch of each switchingdevice being made efiectively relatively insensitive compared to therelays insaid certain branches thereof; so as to beinop'erative for arectified signal output of the detector in that device produced byvoicesignals of normal amplitude in the associatedtransmisslon path but to beoperated in response to a rectified signal output for that detectorproduced by voice "signals ofian amplitude a given amount greater thannormal amplitude in said associated path, "to effectively prevent theecho suppression loss "from being inserted in said associatedtransmission path by operation of the other switching device connectedto the other transmission "path, or to effectively remove that loss ifalready inserted in said associated path.

KINGSBURY H. DAVIS.

